1. Field of Invention
This invention relates to the fabrication of nanocomposites from two or more materials that have low solubility in a liquid milieu and large scale structures of these materials. More specifically fabrication of nano-composites or hybride materials of polymer and graphitic material using nanoparticles as dispersion facilitators is described. In addition, the present invention involves the electromagnetic utilization graphitic material-polymer nanocomposites for supercapacitors and electromagnetic shielding.
2. Prior Art and Overall Description
Fabrication of nanocomposites of two or more materials is not straightforward. Often these materials do not have a common solvent. Even, when both or all materials have a common solvent, their deposition will most probably result into a random structure, in which both components can be clustered as well as mixed in a nanoscale. The present invention solves this problem using carbon nanotubes and cellulose as examples. Efficient dispersion of graphitic materials, such as carbon nanotubes (CNTs) and graphite into polymers continues to be problematic. Graphitic materials tend to aggregate, especially, if their concentration exceeds 10% in the medium. Aggregation prevents the full utilization of the graphitic material. For example, a capacitance of supercapacitor depends on the available surface area of the graphitic material. Aggregate has much less available surface area than individually separated graphitic particles have combined surface area. Electromagnetic interference (EMI) protection efficiency is hampered by clustering. Material strength is much better served by individually dispersed graphitic particles than by clusters.
This invention provides very efficient dispersion methods and materials that are often accompanied by functionalization of graphitic particles. Currently preferred embodiments are almost exclusively related to the fabrication of CNT-cellulose nanocomposite, i.e., graphitic material consists of CNTs, and polymer is cellulose or modified cellulose. Although scanning electron microscope (SEM), and transmission electron microscope (TEM) images prove good dispersion, more importantly, capacitances of supercapacitors, EMI shielding efficacy, and material strength are practically important implications of the efficacy of the dispersion method. Carbon nanotubes (CNTs) can be single walled (SWNT), double walled (DWNT) or multi walled (MWNT). They can have diameters that range from subnanometer to over 100 nm. Also CNTs have a multitude of chiralities. Thus, there are hundreds of different kind of CNTs. Some CNTs are metallic conductors and some are semiconducting. Many CNTs are better electrical conductors than silver at room temperature. All CNTs are very strong, and the tensile strength of the CNTs is tens of times better than that of steel.
CNTs can be functionalized by several methods. One commonly used method is oxidation with the mixture of sulfuric and nitric acids to form carboxylic groups (R. E. Smalley, et al., Method for forming an array of single-all carbon nanotubes and compositions thereof, US2002/0159943 A1). Many moieties can be attached with carboxylic groups. In another method radicals are generated from diazonium salts, and these radicals react with the CNTs (J. M. Tour, et al., Process for functionalizing carbon nanotubes under solvent-free conditions, Int. Appl. WO 2004/007364). In a co-owned method (J. Virtanen et al. PCT/FI2005/000437) CNTs are cut, and the nascent dangling bonds react with a reagent that is present. These are just some examples that are well known in-the-art. Functionalized CNTs are not really CNTs anymore. They will also be called hybride nanotubes (HNTs) in this context. The term HNT-cellulose material will be used all hybride materials of the present invention
CNTs and cellulose are sparingly soluble into all solvents. Due to large particle size they form suspensions rather than solutions, although terms solubility, solution and solubilization will be used here frequently as well as more accurate term dispersion.
CNTs can be solubilized into water using detergents, and into organic solvents as such or using compounds, such as pyrene, that increase the solubility of the CNTs. Detergents may hamper the good properties of CNT-cellulose materials.
Cellulose and its derivatives can be used to disperse CNTs, and cellulose-CNT composites have been used as supercapacitors, and EMI protection, cellulose is: renewable material, and its annual production in the nature might be more than the production of any other natural or man-made polymer. In wood cellulose is intertwined with lignin that is an aromatic polyether. Pure cellulose fibers are few micrometers long, and they have a rectangular cross-section. The cellulose fibers are hollow. Cellulose fibers bind with each other by hydrogen bonds. Because of the large number of hydrogen bonds the interaction is strong. However, water is able to break at least partially the hydrogen bonding network between fibers, and wet paper is very weak.
When nanoparticles or fibers are mixed, composite is formed. If the mixing is efficient, so that the components are mixed also in nanoscale, a nanocomposite will be formed. Fabrication of nanocomposites is not trivial, because often each component tends to separate into clusters that do not contain or contain very little of the other component(s). When the nanocomponents are chemically bound, the material is classified as a hybride material. The main focus of this invention is the fabrication and applications of HNT-cellulose nanocomposites and hybride materials.
It is possible to fabricate cellulose nanofibers that have diameters between 5-100 nm. They are still several micrometers long, and are called microfibrillated cellulose (M. Ankerfors, et al., A manufacturing method for microfibrillated cellulose, 6th International Paper and Coating Chemistry Symposium, 2006).
It is possible to fabricate even smaller bundles of cellulose. Cellulose fragment gel is made of cellulose by cutting the long cellulose molecules shorter and disintegrating the original cellulose fibers into amorphous structures.
One currently preferred method is acid hydrolysis of o-cellulose in order to produce microcrystalline cellulose that has polymerization degree between 200 and 1000 ^jj^-glucose units, preferably between 300 and 500
Another currently preferred cutting method is enzymatic. Cellulose fragment gel forms very strong nanocomposite with CNTs and HNTs. We have found that in the presence of CNTs and nanoparticles, individual molecular fragments of chemically, enzymatically, or biologically fragmented cellulose will be separated and reassembled around CNTs and nanoparticles. This form of fragmented cellulose is currently favored for the dispersion of CNTs. This kind of product is different from other known forms of cellulose composites, and is cellulose-nanoparticle gel, and is called Celose. Cellulose fragments form an amorphous continuum, and the material is mechanically very strong, cellulose fragment-nanoparticle gel is essential for the present invention, and separates it from the prior art that utilizes various forms of cellulose. Carboxymethyl cellulose alone or mixed with cellulose gives good results, but we found that cellulose fragment gel is clearly better. In cellulose fiber hundreds of cellulose molecules are bound together. If cellulose fibers are used as carrier material for CNTs, only the surface of the fibers will be utilized, and the CNT/cellulose mass ratio is small. Despite of small CNT/cellulose ratio the CNTs are not well separated in two directions, although they are excessively separated in third direction (FIG. 1). The situation will be improved, if microfibrillated or nanocellulose will be used. Even nanocellulose has tens of (cut) cellulose molecules in one nanofiber. In cellulose fragment gel molecules are equally separated in all directions, and separation can adjusted at molecular accuracy by choosing the desired CNT/cellulose ratio. Cellulose fragments interact individually with CNTs by wrapping around the CNTs.
Thus, the CNT/cellulose ratio will be maximized. Although nano-, and microparticles are useful for the dispersion they are not mandatory, especially for the short chained cellulose fragments, such as some forms of microcrystalline cellulose. Organic ionic solvents are good solvents for both cellulose and CNTs. Their high price is a serious drawback for several practical applications. Also during various deposition processes some CNTs and cellulose molecules form homoaggregates. The same problem is encountered with many other methods, in which cellulose is solubilized. When molecular components are well solvated, their mutual interaction in solution is suppressed. While solubilized cellulose is not properly wrapping CNTs, and deposition results at least into partial phase separation. When the deposition is induced by an outside effect, the product tends to be under kinetic control. In the method of present invention two water insoluble solids, fragmented cellulose and CNTs, are mixed in water using uneven distribution of kinetic energy that is able to separate cellulose fragments as well as CNTs from their own fibers and clusters. Cellulose molecular fragments will wrap around individual CNTs under thermodynamic control. Advantageously, nano- or microparticles will be used to enhance the effect of the kinetic energy. The resulting CNT-cellulose nanocomposite is stable enough so that the components do not separate under the conditions that are used in this invention.
The situation with various forms of cellulose is somewhat analogous to various forms of iron, such as chemically pure iron, cast iron, steel, stainless steel, and acid resistant steel. There are several subspecies of each of these. They have different mechanical properties, malleability, molding properties, chemical resistance, electrical conductivity, and magnetic properties. In some applications it might be possible to choose between two or more kinds of iron. However, in many applications only one kind of iron may be used. Similarly, we have found that out of several kinds of cellulose and its derivatives, cellulose nanoparticle gel (Celose) is best for the electrical applications of this invention, although some other forms of cellulose can be used with partial success.
In a co-owned patent application is described the fabrication of CNT-cellulose nanocomposite that contains also electrically conducting nanoparticles (J A. Virtanen and P. Moilanen, WO/2008/034939). Combination of CNTs and electrically conducting nanoparticles is not enough. For several applications, including supercapacitors and EMI protection, there must be a good electrical contact between CNTs and nanoparticles. Mixing of the components even at nanoscale does not guarantee always the good electrical contact.
The composites of the present invention are also based on CNT-cellulose nanocomposite that contains electrically conducting nanoparticles. In the present invention the methods and compositions will be provided that ensure good electrical contact between CNTs and electrically conducting nanoparticles. Thus, the full utility of electrically conducting nanoparticles is obtained.
This nanocomposite may contain also paramagnetic particles, such as magnetite particles, or these particles may be advantageously in an adjacent layer. Thus, the macrostructure will contain both electrically and magnetically active particles providing superior protection against electromagnetic interference (EMI protection).
Magnetic particles can be made from iron, nickel, or cobalt, for example. Paramagnetic particles are typically ferrites. Magnetite is one specific example.
One major class of supercapacitors utilizes carbon nanoparticles (CNPs). Because CNPs have poor electrical conductance, only thin layers can be used. Similarly one class of EMI protecting materials is based on CNPs. CNPs give a random structure that can not be the best possible structure. More control of the structure is obtained, if CNTs or graphene will be used. However, so far the results have been disappointing mainly because CNTs and graphene tend to aggregate.
Prior art, all of which are given here as reference in their entirety:    M. Ankerfors, et al., US Patent Application 20090221812
Method for the Manufacture of Microfibrillated cellulose
cellulose was enzymatically (endoglucanase) cut, refined, and homogenized with high pressure fluidizer/homogenizer. The resulting product is micro- (or nano) fibrous cellulose that is different from the product of the present invention. Although the methods resemble each other, the different product can be explained by the presence of CNTs and/or nanoparticles in the methods of the present invention. CNTs and nanoparticles bind individual cellulose molecules and prevent their recombination into fibers after they have been separated even temporarily.    J. Engelhardt, et. al., WO 2009/021687 A 1
Nanoparticles of amorphous cellulose
Fabrication of amorphous cellulose nanoparticles is described by a method that resembles that of Ankerfors et al. except that chemical cleaving of cellulose is used instead of enzymatic cleaving. However, the method of Engelhardt et al. provides amorphous nanoparticles instead of nano- or microfibers. One reason is shearing and ultrasonic vibration of fragmented cellulose. In the present invention the formation of cellulose particles, even nanoparticles, is undesirable. The formation of cellulose particles is avoided, because cellulose molecules wrap around of CNTs and nanoparticles.    H. Tennent, et al., U.S. Pat. No. 856,657
Graphitic nanofibers in electrochemical capacitors.
The capacitor in which nanofibers are coated with a thin coating layer of carbonaceous polymer. Polymer can be cellulosic polymer. However, no example is given.    C. Niu, et al., Appl. Phys. Lett. 70 (1997) 1480
CNT electrodes for high power electrochemical capacitors    P. Glatkowski, et al., U.S. Pat. No. 6,265,466
EMI shielding composite containing CNTs and polymer is described. Disclosure and examples limit the concentration of CNTs to 15%. Higher concentration require new methods, such as described in the present invention that allows any concentration between 0 and 100%. Glatkowski et al. require also orientation of CNTs by a shearing force. The present invention provides so good dispersion that enough CNTs will be oriented into every direction for EMI protection.    W. Li, H. Probstle, and J. Fricke, J. Non-Crystalline Solids 325 (2003) 1.
carbon aerogel supercapacitor is described.    C. H. Cooper et al., US Patent Application 20050272856 Jul. 7, 2004
CNT containing materials and articles containing such materials for altering electromagnetic radiation,
Claims are limited for the case, in which the length of CNTs is more than half the wavelength. In the present invention this requirement is not necessary. As a matter of fact the CNTs that are one hundredth of this limit work well, when the present method is used. This will make the materials of the present invention much more economical.    P J. Glatkowski, et al., U.S. Pat. No. 7,118,693 Oct. 10, 2006
Conformal coatings comprising CNTs.
CNT diameter is claimed to be smaller than 3.5 nm that essentially requires that the CNTs are SWNTs. Very low loadings were used that are far lower than optimal concentration. One reason that can be the inefficient dispersion of CNTs over 1% and difficult dispersion over 10%. cellulose is mentioned as a possible polymer, but is not described in Experimental section. Sonication of CNTs into toluene solution, and mixing this solution with epoxy, polyurethane, acrylic, or silicone coatings was described by Glatkowski et al. cellulose was not an example. Repeating the method of Glatkowski et al. for cellulose was performed by the inventors of this invention. CNTs were dispersed either into water or 2-propanol in the absence or presence of cellulose. One typical example is in FIG. 20. The results were very different from those of the present invention. In FIG. 20 cellulose fibers are virtually intact (FIG. 20 A), and CNTs are lying on the surface of cellulose fibers (FIG. 20 B, schematics in FIG. 1 A). Thus, the EMI shielding efficacy obtained by Glatkowski et al. is limited by dispersion method.    P J. Glatkowski, et al., U.S. Pat. No. 7,118,693
Conformal EMI shielding coating is described. CNTs have outer diameter 3.5 nm or less, while in the present invention the diameter is 3.5 nm or more. Glatkowski et al. require also insulating layer. While one embodiment of the present invention also contains CNT-polymer layer and an insulating layer that layer contains magnetically active particles. A three layer structure of the present invention has an insulating layer sandwiched between two CNT-polymer layers, and destructive interference of electromagnetic radiation is created between two CNT-polymer layers. This is totally new effect that is not described by Glatkowski et al.    C. Du and N. Pan, nanotechnology 17(2006)5314
Electrophoretic deposition of carbon nanotubes on electrodes is described.    C.-C. Hu, et al., J. Phys. Chem. Solids 68 (2007) 2353.
Modification of MWNTs for electric double-layer capacitors.    S. Yoshimitsu, U.S. Pat. No. 7,382,601 Jun. 3, 2008
Electric double layer capacitor and method of manufacturing same.
Incorporating fullerene into CNT electrodes by microwave radiation is described.    J. S. Douglas, US Patent Application 20080044651
A coating or ink comprising CNTs is described. Dispersion may also contain nanoparticles and polymers including cellulose, cellulose is only mentioned by name without further definition or enabling embodiments. Also surface roughness is required to be about 100 nm or less. This is almost impossible to achieve with a mixture CNTs and cellulose, because both are fibrous solids.    W. Lu and H. Kent Douglas, US Patent Application 20080192407
Ultracapacitors with carbon nanomaterials and ionic liquids.
Ionic liquids are used as electrolytes. Polymers are carrier materials so that the supercapacitor is largely solid. Inorganic material is used to prevent crystallization of polymeric material.    P. M. Ajayan, et al., US Patent Application 20080212261
Energy storage devices and composite articles associated with the same.
Method claims of Ajayan et al. require polymer is dissolved into a liquid. That is in sharp contrast of the present invention, in which the polymer is advantageously poorly soluble into the liquid. Importantly, no kinetic energy input is mentioned in disclosure or in any embodiments.    US Patent Application 20080254362
Nano-Composite structures, methods of making, and use thereof.
Nanocomposite containing CNTs and semiconducting or metallic nanoparticles is described. Also polymeric binder may be included. Apparently no polymers are defined in the disclosure, and no examples of their use is given. However, several specific polymers are claimed, but cellulose is not included.    U.S. Pat. No. 7,553,341 Jun. 30, 2009
High power density supercapacitors with carbon nanotube electrodes.
Electrophoretic deposition of CNTs.    J. S. Glatkowski, et al., US Patent Application 20090131554
EMI shielding CNT-polymer composite is provided. In strong contrast with present invention the composite has low or essentially no bulk conductivity. Glatkowski et al. realized that entanglement of CNTs is essential for good EMI protection. Entanglement is difficult to obtain with high loadings. This problem has been solved in the present invention, and high concentrations of CNTs can be used, and CNT-polymer nanocomposite has also good bulk conductivity.    K.-L. Jiang and S.-S. Fan, US Patent Application 20090168302
Electrochemical capacitor with CNTs.
Use of cellulose in porous middle membrane is mentioned. However, Membrane is not integrated with CNT layers.    J.-H. Kwon, et al, U.S. Pat. No. 7,588,700
Specific EMI shielding material comprising epoxy or polyimide resin and CNTs that contain both C=0 and N—H functionalities. Composition may further comprise metal particles.    A. G. Rinzler, et al., U.S. Pat. No. 7,704,479
Highly accessible, nanotube electrodes for large surface area contact applications.
Fabrication of porous CNT-membrane is described. Sacrificial nano- or microparticles are used. These particles will be removed after the fabrication of a film. Dispersion method is not specified, but the requirement is that CNTs and particles do not flocculate from suspension. This is in sharp contrast to the present invention, in which the CNT-polymer-nanoparticle nanocomposite flocculates advantageously from suspension. Also in the present invention nano- or microparticles will be used to focus kinetic energy, and they must have density that is advantageously twice that of the solvent. Rinzler et al. use nano-, and microparticles only as spacers, and the density is not important, unlike in the present invention.